This application claims the priority of Korean Patent Application No. 10-2006-03097, filed on Jan. 11, 2006, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a semiconductor integrated circuit, and more particularly, to a temperature sensor using a ring oscillator and temperature detection method using the same.
2. Description of the Related Art
Semiconductor devices have temperature characteristics in terms of operations. Typical operational characteristics of semiconductor devices are consumption current IDD and operation speed tACCESS. FIG. 1 is a graph of temperature characteristics of a semiconductor device. Referring to FIG. 1, as the temperature increases, the operation speed increases (A) and the amount of the consumption current IDD decreases (B).
These temperature characteristics are of great importance to volatile memory devices such as dynamic random access memory (DRAMs). Since DRAM cells have an increase in the amount of leakage current as temperature rises, data maintenance abilities can be deteriorated due to charges, which reduces data maintenance time. To address this issue, DRAMs require a faster refresh operation. That is, it is necessary to provide a method of changing the refresh period of DRAMs according to temperatures because of the effect on data maintenance capabilities higher temperature changes has. To this end, a temperature sensor is required to sense the temperature inside DRAMs.
FIG. 2 is a circuit diagram of a conventional temperature sensor. Referring to FIG. 2, the temperature sensor 200 comprises a proportional to absolute temperature (PTAT) current generator 210, a complementary to absolute temperature (CTAT) current generator 220, and a comparator 230.
The PTAT current generator 210 comprises first and second PMOS transistors MP1 and MP2, first and second NMOS transistors MN1 and MN2, a resistor R, and first and second diodes D1 and D2. The first and second PMOS transistors MP1 And MP2 have the same size and include a first current mirror. The first and second NMOS transistors MN1 and MN2 have the same size and include a second current mirror. The size of the first and second diodes D1 and D2 has a ratio of 1:M.
Since the first current mirror of the first and second PMOS transistors MP1 and MP2 and the second current mirror of the first and second NMOS transistors MN1 and MN2 are symmetrical, amounts of currents Ia1 and Ia2 are identical to each other. That is, Ia1:Ia2=1:1.
A turned-on current ID of a general diode is indicated below,ID=Is*(eVD/VT−1)=Is*(eVD/VT)  (1)
wherein, Is denotes a contrary directional saturized current of the diode, VD denotes a diode voltage, and VT is a temperature voltage indicated as kT/q. Therefore, the current Ia1 flows through the first diode D1 as indicated below,Ia1=Is*(eVD1/VT)  (2)
A first diode voltage VD1 isVD1=VT*ln(Ia1/Is)  (3)
A second diode voltage VD2 isVD2=VT*ln(Ia2/(Is* M))  (4)
Since the amounts of the currents Ia1 and Ia2 are identical to each other, the first diode voltage VD1 is almost the same as a present temperature voltage NOC0. Therefore,V(NOC0)=VD1=VD2+Ia2*R,  (5)
If equations 3 and 4 are substituted for equation 5,VT*ln(Ia1/Is)=VT*ln(Ia2/(Is*M))+Ia2*R  (6)
Therefore, the current Ia2 isIa2=VT*ln(M)/R  (7)
Thus, the current Ia1 is proportional to a temperature. That is, the PTAT current generator 210 generates the current Ia1 proportional to a current temperature.
The CTAT current generator 220 comprises a third PMOS transistor MP3, a third NMOS transistor MN3, a plurality of resistors Raa, RU1˜RU5, and RD1˜RD5, and a plurality of switching transistors TU1˜TU5 and TD1˜TD5.
The third NMOS transistor MN3 connects to first and second NMOS transistors MN1 and MN2 and a current mirror. An amount of a current Ib is almost identical to the amounts of the currents Ia1 and Ia2. The switching transistors TU1˜TU5 and TD1˜TD5 are selectively turned on/off in response to trip temperature control signals AU1˜AU5 and AD1˜AD5, so that the resistors RU1˜RU5, and RD1˜RD5 connected to the turned-on switching transistors TU1˜TU5 and TD1˜TD5 are selectively short-circuited.
If the amounts of the currents Ib, Ia1, and Ia2 are almost identical to one another, VA and VB node voltages of the PTAT current generator 210 are almost the same as a VC node voltage of the CTAT current generator 220. In equations 3 and 4, a VT voltage increases as the temperature increases; however, an amount of the current Is increases greater than the amount of the VT voltage. A diode voltage is reduced as the temperature decreases. Therefore, an amount of the current Ib that flows through the resistors Raa, RU1˜RU5, and RD1˜RD5 is reduced as the temperature increases. That is, the current generated by the CTAT current generator 220 is in inverse proportional to the temperature.
The comparator 230 compares the present temperature voltage NOCO and a sensed temperature voltage NOC1. The present temperature voltage NOCO and a sensed temperature voltage NOC1 are determined using the current Ia1 and the current Ib, respectively. The temperature sensor 200 detects a present temperature at a point where the amount of the currents Ia1 and Ib are identical to each other as illustrated in FIG. 3. FIG. 3 is a graph explaining the temperature detection method using the temperature sensor illustrated in FIG. 2.
Referring to FIG. 3, the current Ia1 is proportional to the temperature, whereas the current Ib is in inverse proportional to the temperature. For example, when a present temperature of a chip including the temperature sensor 200 is 45° C. If the amount of the Ib current is less than the amount of the current Ia1, the trip temperature signals AU1˜AU5 and AD1˜AD5 of the CTAT current generator 220 are selectively enabled to control a resistance value of the CTAT current generator 220 and to flow a great amount of the current Ib (C), so that the amounts of the currents Ib and Ia1 are substantially identical to each other.
To the contrary, if the amount of the Ib current is greater than the amount of the current Ia1, the trip temperature signals AU1˜AU5 and AD1˜AD5 of the CTAT current generator 220 are selectively disabled to control the resistance value of the CTAT current generator 220 and to flow a small amount of the current Ib (D), so that the amounts of the currents Ib and Ia1 are identical to each other. If the amounts of the currents Ib and Ia1 are identical to each other, the temperature sensor 200 senses the present temperature of the chip, i.e. 45° C.
However, the temperature sensor 200 uses a bipolar transistor of an NPN transistor or a PNP transistor in order to realize the first and second diodes D1 and D2. The NPN transistor or the PNP transistor has analog operational characteristics, where the temperature sensor 200 may sense a nonlinear change in the temperature of the chip. Also, because both the NPN transistor and the PNP transistors are large, their inclusion increases the area of the chip.